1. Field of the Invention
This invention relates to cameras, and particularly to release devices therefor. The invention is especially directed to cameras in which depression of a shutter release button to a first position or step connects the camera circuitry to a power source and actuates a range finder as a preliminary setting for photography, and in which depressing the shutter to a second position or step focuses the lens in response to the range finder, sets the exposure, and starts subsequent actions. Depression to the second position thus serves as a trigger for photography.
2. Description of the Prior Art
In general, photographic operations in a camera are carried out in response to operation of a so-called release button, shutter release button, or shutter button. Along with recent advances which involve electronics for a camera, many types of cameras use the shutter button to connect the circuits to a power source, and to initiate the processing of various types of data such as range finding before the photographic operation. In such a camera, the shutter button is depressed to a first position for connecting the circuitry to a power source and performing pre-photographic actions, and to a second position for effecting the photographic action, such as control of the lens position and setting the exposure. That is, the power source connection and preliminary photographic actions are carried out by depressing the release button to its first position in response to which the range finding operation is confirmed in the display, the results of the range finding operation are displayed, and the exposure data are displayed. Control of the lens position, exposure of the film, and if necessary the film winding are accomplished by depressing the release button to the second position.
Various types of digital electrical circuits or microcomputers have been built into cameras. The general tendency is for such systems to digitally carry out exposure computation and range finding operations. However, digital types of processing systems have the inherent shortcoming of requiring finite lengths of time for processing data. Increases in the cost or increases in the power consumption would be unavoidable for speeding up the time needed to process the data. Data processing for range finding requires complicated operating routines and results in the need for a comparatively long period of time. Also, in systems employing CCD's for range finding, such as the range finding system disclosed in U.S. patent application Ser. No. 298,532, filed Sept. 1, 1981 by the Assignee of the present application, requires a finite length of time for accumulating the charge in the CCD's and for checking the length of time. When the scene is dark, the length of time needed may sometimes be as long as 0.5 seconds. In some cases, a range finding completion signal will not be produced.
When such a range finding system is built in a camera, as described in the aforementioned U.S. patent application Ser. No. 298,532, the range finding and accompanying computation as well as exposure measurement are performed in response to the shutter button reaching the first position or step. Control of the lens position as well as subsequent photographic operations are performed when the shutter button reaches the second stroke. Here, it is essential that the distance measured by the range finding device be available for effecting control of the lens position. Therefore, an operator is limited in using this type of camera because the operator must wait for completion of the range finding process after the shutter button reaches the first step, and only after the range finding is completed and the results thereof confirmed by an indication within a viewfinder or by sound, depress the button to the second position for effecting the remaining photographic operations. However, often an operator will perform the first stroke and the second stroke in a single action and disregard the above limitation. In this case, the lens focusing may be performed without the results of the range finding operation. This introduces a great risk in that the lens will not reach its prescribed position and will therefore be out-of-focus when the picture is taken.
In order to overcome these disadvantages, the present inventor has proposed that when the second position is reached in depressing the shutter button while the range finding result has not been obtained after the first position, the lens is moved only after range finding operations have been done for a prescribed number of times. However, this arrangement makes the set up complicated and demands for more economical systems have been rather keen.
In U.S. Pat. No. 4,219,260, issued on Aug. 26, 1980, the present inventor discloses countermeasures for incorrect exposure measurements due to premature depression of a shutter button which shortens the photosensing time. This serves to control the length of time required for an operating signal to be generated by an exposure control mechanism corresponding to the magnitude of an output of a photosensing circuit for stabilizing the operation of the photosensing circuit and thereby obtain an appropriate exposure. However, the aforementioned proposal fails to concern itself with the problem caused by premature depression of the shutter button with respect to range finding.